Calcineurin activity is required for the initiation of skeletal muscle differentiation

Differentiation of skeletal muscle myoblasts follows an ordered sequence of events: commitment, cell cycle withdrawal, phenotypic differentiation, and finally cell fusion to form multinucleated myotubes. The molecular signali ng pathways that regulate the progression are not well understood. Here we investigate the potential role of calcium and the calcium-dependent phospha tase calcineurin in myogenesis. Commitment, phenotypic differentiation, and cell fusion are identified as distinct calcium-regulated steps, based on t he extracellular calcium concentration required for the expression of morph ological and biochemical markers specific to each of these stages. Furtherm ore, differentiation is inhibited at the commitment stage by either treatme nt with the calcineurin inhibitor cyclosporine A (CSA) or expression of CAI N, a physiological inhibitor of calcineurin. Retroviral-mediated gene trans fer of a constitutively active form of calcineurin is able to induce myogen esis only in the presence of extracellular calcium, suggesting that multipl e calcium-dependent pathways are required for differentiation. The mechanis m by which calcineurin initiates differentiation includes transcriptional a ctivation of myogenin, but does not require the participation of NFAT. We c onclude that commitment of skeletal muscle cells to differentiation is calc ium and calcineurin-dependent, but NFAT-independent.